This invention relates to a device for measuring the amount of a first material dispersed within a second material and more particularly to the measurement of a binder dispersed in a mat of fibrous material such as a mat of glass fibers. The invention may be used to measure the moisture content in the binder or it may be used to measure any other constituent of the binder such as a solid material suspended in a liquid. It also may be used to measure a change in the amount of a constituent in the binder to determine the cure state.
Instruments for measuring moisture content, for example moisture within a fibrous body, are well known in the art. For example, U.S. Pat. No. 3,851,175 shows a moisture gauge for measuring the moisture content of a moving sheet of paper. Specifically, it uses radiated energies at two closely related wave lengths, a first wave length L.sub.1 and a second wave length L.sub.2. L.sub.1 is a reference and is relatively insensitive to the moisture within the paper. L.sub.2 is sensitive to the presence of moisture and is absorbed by the moisture within the paper. The energies at these two wave lengths are measured, after transmission through the paper, and an energy ratio is taken and converted to moisture by a calibration procedure. In actual use, a calibration curve is drawn wherein samples of predetermined moisture content are radiated and a ratio is taken. These ratios are plotted against the actual moisture content for use as a calibration curve.
Other devices known in the prior art add improvements to this basic idea. For example, U.S. Pat. No. 4,097,743 teaches the use of a set of two closely related wave length radiated energies to derive a ratio of transmitted energy through a material and further teaches the use of a second set of the two closely related energies directly transmitted without passing through the material for a control in determining the effect of temperature induced optical transmission changes and to cancel any variable as transmission changes from the moisture measurement.
Another U.S. Pat. No. 4,006,358 uses the same measurement technique and adds a filter which compensates for a change in optical transmission due to dirt collected on the instrument.
Still another U.S. Pat. No. 3,405,268 uses three sources at closely related wave lengths, a first wave length responsive to the moisture in a paper web, a second wave length not responsive to the moisture or the cellulose in the paper, and a third wave length that is responsive only to paper. The transmitted energy through the material at each of these wave lengths is detected and a first ratio of the energies at the first and second wave lengths is taken to determine the moisture in the web. A second ratio of the energy transmitted through the material at the third wave length to the energy at the second wave length is taken to determine the proportional cellulose content of the paper and the basis weight. These two ratios are then taken together to indicate the moisture content and the basis weight.
Of particular interest is U.S. Pat. No. 3,851,175 which shows the use of an opacity signal factored in with the ratio signal indicative of the transmitted energies at the two closely related wave lengths to indicate moisture. As described in that patent, the use of a ratio at the two closely related wave lengths does not provide a true indication of moisture content in paper where the content of the paper may vary especially due to different types of wood and wood mixtures used in making the paper. As described in that patent, it is necessary to use an opacity signal to determine the opacity or the degree to which energy at the energies may be transmitted through the paper. This opacity signal then is used in recognition of the special problem in measuring moisture content in paper. One problem presented by paper is the opacity changes due to the different mixture of woods in the paper. As explained, the variations in opacity of paper is due to the variations in the pulp fiber mix and produces variations in the signals indicative of the transmitted energy through the paper, ranging up to two orders of magnitudes. Therefore, the opacity signal is derived as the amplifier gain needed to maintain at a constant voltage the reference channels of the two related wave length energies so that their relationship is accurately determined and the ratio of the signals does not vary except due to moisture. Automatic gain control is in response to the variations in the opacity of the paper to maintain the amplification of the two channels for the two radiated energies so that the two signals and their ratio is indicative of the moisture content. Additionally, the opacity signal is used to correct the linear slope of the prederived plot of moisture versus the signal level ratio. It is used to adjust the ratio amplitude indicative of the moisture relative to different hardwood-softwood proportions with identical basis weights.
The prior art then shows the technique of using radiated energy at two closely related wave lengths, measuring the signal levels of energy transmitted through a sample at the two wave lengths and taking the ratio therebetween to establish moisture content. Further shown, particularly for use with determining moisture content in paper, are techniques for correcting the ratio measurement for the opacity of paper due to the variations in the softwood-hardwood pulp mixture as well as for dirt accumulated on the instrument which changes the effective intensity of the radiation.
However, the teachings of the prior art are limited to the special problem presented by changes in opacity of the sample and the effect of those changes on the measurement of moisture. The prior art techniques also are limited to measurement of moisture in wood pulp. The prior art measures the transmitted radiation through the moisture contained by the pulp or web. The radiation is attenuated by the moisture and the web. The ratio amplitude is altered to bring it into correspondence with a precalibrated curve for moisture within a sample web or pulp moisture of the same pulp consistency.
The prior art does not deal with nor teach the measurement of materials other than moisture within a fibrous body or change in amount of the materials as an indication of process state or material state, nor does it teach the solution of the problem presented by changes in weight upon the primary measurement or the manner in which the ambiguity introduced by the weight of the material into the primary measurement may be analyzed and the measurement corrected.